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Someone alone on an island would never come up with anything like the stories in the bible. (That's why different cultures have completely different religions.) How are people supposed to know this stuff if they don't read the bible and don't talk to anyone else who has read the bible? Where else do these ideas come from?

"For since the creation of the world God's invisible qualities--his eternal power and divine nature--have been clearly seen, being understood from what has been made, so that people are without excuse." http://biblehub.com/romans/1-20.htm